Formation, Growth, and Shrinkage of Voids in Lithium Metal in Contact With an LLZO Electrolyte

A leap forward in the energy density of solid-state batteries necessitates the successful implementation of metal electrodes. A serious problem of metal electrodes is their non-uniform deposition and dissolution, which can lead to dendrites and voids. Here, we report on formation, growth, and shrinkage of voids based on operando SEM observations of lithium metal in contact with a Li7La3Zr2O12 (LLZO) electrolyte. Our observations reveal a general tendency for the replenishment of voids upon redeposition of lithium. We explain this by gradients in the mechanical stress that act as driving forces for atom motion inside the electrode and thus feed lithium away from the interface back into existing voids, which restores the original shape of the electrode. During operation, lithium atoms are inserted or extracted at the interface, creating stresses in the electrode. In electrodes with smaller lithium grains we observe that the shape of voids is strongly influenced by the microstructure of the lithium metal and that grain boundaries can obstruct void growth. We consider diffusion along lithium grain boundaries and the inner surface of voids as important pathways for metal transport. We suggest that the local balance of the diffusional metal fluxes is responsible for the evolution of the shape of the metal electrode.